Shank For Bone Implants

Abstract

The shank is provided with texturized zones in the external surface to insure uniform contact between the bone cement and the shank when the shank is implanted. The texturized zones allow the shank to be withdrawn readily from a bone and can be formed to depths of 10 to 100.mu..

Description

This invention relates to a shank for a bone implant.

Bone implants, such as joints for the hip, elbow and wrist, usually have shanks which are engaged in corresponding passages in the particular bone concerned and are anchored in the passage by means of a quick-setting bone cement such as methylmethacrylate. It has been found that the bone cement, while curing, often changes volume either by increasing or, and more frequently, by contracting. Because of this, and particularly where the volume changes occur lengthwise of the shank, the volume changes are very often the cause of the bone cement becoming detached from some parts of the shank surface. The parts affected then cease to be able to transmit any load from the shank to the bone. This, in turn, leads to recession of the bones in the parts concerned, so that the shank starts to work loose. The load also tends to become increasingly concentrated in the remaining unloosened parts of the shank, often to such an extent that the shank ruptures.

A main reason for the above problems is that present-day bone cements cure rapidly and are relatively highly viscous plastics with poor contact properties, i.e., the cements do not readily form an accurate "negative" of the shank surface.

Daily practice with bone implants and their shanks has also shown the need for the implant and the anchorage of the implant to be so devised that the implant can be withdrawn readily from the bone at any time without damage to the bone surrounding the shank.

Accordingly, it is an object of the invention to provide a shank for an implant which is capable of compensating for changes in volume in a bone cement during setting.

It is another object of the invention to preclude the loosening of bone implant shanks in a bone due to changes in the volume of a bone cement.

Briefly, the invention provides a shank having texturized zones of depressions in an external surface for anchoring a bone implant in a bone. The zones are generally disposed in the load bearing zones of the surface of the shank while the shank narrows in cross-section toward one end.

In use, the shank is secured in a bone by a suitable bone cement so that the cement fills the spaces within the texturized zones of depressions.

Advantageously, the depth of the depressions in each zone is between 10 and 100 microns (.mu.). Shallower depths lead to unreliable adhesion between the cement and the shank surface, and depths of more than 100 microns (.mu.) readily result in air bubbles remaining when the shank is driven into the bone cement, the bubbles impairing adhesion. Also, the possibility of withdrawing the shank from the bone is impaired in increasing proportion as the depressions are made deeper. The optimum depth is governed very largely by the particular bone cements used, more particularly their viscosity, changes in volume and their ability to make good contact, i.e., the extent to which they can mate accurately with a texture on the shank surface to provide a faithful copy "in reverse" of the texture.

In order to ensure satisfactory adhesion, at least the bearing and load-taking surfaces are provided with the texturized zones of depressions. For example, in one embodiment these surfaces are provided with regular toothing extending transversely of the direction in which the implant is introduced into and withdrawn from the bone. In this event, to facilitate withdrawal, the distance between discrete teeth can increase towards the narrow end of the shank.

Textures in the form of recesses of a shape resembling shells or troughs have proved satisfactory in other embodiments of the invention. Both kinds of texture can, of course, be used with advantage simultaneously. For instance, in the case of a multiple polygonal shank, the load-taking and the bearing surfaces can have toothing while the other surfaces can have shell-like recessings. To further improve adhesion, the texturized shank surface can also be roughened, e.g. by sand blasting with 0.05 to 0.2 millimeters (mm) diameter silica pellets.

Conveniently, to improve the fatigue strength of the shank, the edges and corners at junctions between the various surfaces which make up a multi-surface polygonal shank, and the raised and recessed junctions between the various teeth and between and in the troughs, are very rounded and free from projections.

These and other objects and advantages of the invention will become more apparent from the following detailed description and appended claims taken in conjunction with the accompanying drawings in which:

FIG. 1 illustrates a sectional view through a femur fitted with a prosthetic head having a shank according to the invention;

FIG. 2 diagrammatically illustrates a sectional view through an arm fitted with an artificial elbow joint using shanks in accordance with the invention;

FIG. 3 diagrammatically illustrates a sectional view of a prosthetic wrist joint fitted in a forearm and a carpal bone using shanks according to the invention;

FIG. 4 illustrates a considerably enlarged and diagrammatic plan view of a texturized zone of depressions in the form of regularly disposed trough-like recesses;

FIG. 5 illustrates a vew taken on line V--V of FIG. 4;

FIG. 6 illustrates a side elevational view of a cutting bit for forming the depressions shown in FIGS. 3 and 4; and

FIG. 7 illustrates a plan view of the bit of FIG. 6.

Referring to FIG. 1, a prosthetic femur head 1 has been implanted in a femur 2 which has been appropriately prepared by surgery. The head 1 includes a shank 3 anchored by means of a bone cement 4, e.g. methyl methacrylate, in a passage or recess 5 in the femur bone 2 whose relatively compact cortical substance is shown more darkly dotted than the porous spongy substance. In the part near the head 1, the shank 3 has a polygonal large-area shape with rounded corners and edges. In order to prevent accidental turning, the shank cross-sectional shape resembles a kite (FIG. 6) and in the end-distal from the exposed end merges, on the assumption that the shape narrows on all sides and continuously, into a substantially trapezoidal cross-section having rounded edges.

The shank loosening previously referred to occurs, in the artificial hip joint shown, more particularly in region A of bearing or support surface 6, to spread in the course of time more particularly to region B of load-taking surface 7.

The surface of the shank 3 which is non-porous has a texturized zone which is, indicated in FIG. 1 by trough-like discrete unconnect recesses or depressions between teeth of a regular or uniformly formed toothing. An endeavor is also made to show, in diagrammatic and sketch form, the toothing formed as texturizing on the surfaces 6, 7 which appear just as section lines. Also indicated is an increase in tooth spacing towards the exposed end of shank 3.

Referring to FIG. 2 as another example of implant shanks having texturized surfaces, an elbow joint 8 has a shank 9 on one part which shank 9 narrows on all sides. The shank 9 is introduced into a recess or passage or the like in a humerus 10 and retained by bone cement 4. A shank 11 on the other part of the joint 8 is anchored similarly in an ulna 12.

Referring to FIG. 3, for a wrist joint 13, one shank 14 is retained by bone cement 4 in a radius bone 25 and another shank 26 is anchored by bone cement 4 in a carpal bone and/or metacarpal bone 27.

Referring to FIGS. 4 and 5 which are views to a considerably enlarged scale, the texturizing can take the form e.g. of trough-like or shell-like recesses or depressions 15 disposed regularly in rows a, b, c. If the raised parts between the discrete shells 15 are also removed inside the various rows a, b, c, the resulting toothing is very advantageous for the surfaces 6 and 7. The advantages of toothing are that, in cooperation with a shank shape that narrows on all sides and continuously, a fitted shank can be removed simply by applying a force sufficient just to release the shank by one toothing step in the withdrawal direction. Thereafter, the shank can readily be withdrawn from the cement bed.

Referring to FIG. 5, the shank material 16 is usually one of the known metal alloys which are conventionally used for bone implants. As already emphasized, the depth t of the depressions in the texturized zones can be between 10 and 100 .mu.. From 20 to 30 microns (.mu.) has been found a very good value for the depth t in existing shanks where methyl methacrylate is used as the bone cement. Experiments by the Applicant has also shown that satisfactory dimensions for a single shell of shell-like texturizing are given by a ratio of length u to width v to depth t of 12 : 4 : 1.

The surface 17 of the texturized shank 3 has the texturizing roughened as well, to further improve adhesion between the cement 4 and the shank 3. That is, the portions between the depressions 15 which are rounded so that the depressions merge together smoothly, can be roughened. As mentioned, this roughness is produced by blasting with silica pellets of from 0.05 to 0.2 millimeters (mm) diameter.

A technique which has proved satisfactory for producing the shell-like texturing, is the cutting or milling of the periphery of the shank transversely of a longitudinal direction. A suitable tool 20 for this can be seen in FIGS. 6 and 7. The cutting bit has, at the junction between the cylindrical portion and the circular end face, a radius r of curvature which is adapted to the bit diameter d and to the required trough size. Also, the bit has a central plane surface s which is also of a particular diameter. As FIG. 7 shows, the bit 20 has six lips 21 a, 21 b. Two lips 21 a are disposed opposite one another to cover the whole bit along a diameter, whereas the other four lips 21 b extend only to the edge of the plane surface. While being texturized, the shank 3 is positioned relatively to the cutter bit 20 in the manner visible in FIG. 6.

As already mentioned, the trough depth t arising from the cutting or milling operation is mainly governed by the cutting radius r. The length u can be controlled to some extent by cutter speed and/or the rate of feed, and the width v can be varied within limits by varying the "line spacing" of the cutting operation. The same tool 20 can be used to produce toothing, but instead of cutting being performed by the cutting edges of radius r, the cutting is performed by the side walls of the cutting edges.

As an example, for a texturizing of the kind specified with the ratio of u to v to t .apprxeq. 12 : 4 : 1 and with t to .apprxeq. 20 microns (.mu.), the cutter 20 has the diameter d of 8 millimeters (mm), the radius r of curvature of approximately 3.25 millimeters (mm) and the diameter of the plane surface s of approximately 1.5 millimeters (mm). The feed in this case is 250 mm/min. and cutter speed is approximately 8 meters/minute (m/min).

The texturizing can also be formed in the shank surface e.g. by means of forming punches.

Claims

What is claimed is:

1. A solid shank for implanting in a bone cement to anchor a bone implant in a bone, said shank having a narrowing cross-sectional area towards one end and an external non-porous surface having discrete unconnected depressions therein between raised portions of said shank, said depressions being between 10 microns and 100 microns depth.

2. A shank as set forth in claim 1 wherein said depressions are separated by rounded portions to merge together smoothly.

3. A shank as set forth in claim 1 wherein said depressions extend into said surface for a depth of between 20 microns and 30 microns.

4. A shank as set forth in claim 1 having a longitudinal axis and wherein said surface includes at least two load bearing zones having said depressions therein, each zone having a regular toothing extending transversely of said longitudinal axis to form said depressions.

5. A shank as set forth in claim 4 wherein said regular toothing includes transverse rows of spaced apart teeth with the spacing between discrete teeth increasing toward said one end of said shank.

6. A shank as set forth in claim 1 wherein said zones includes a plurality of regularly disposed trough-shaped depressions.

7. A shank as set forth in claim 1 having a plurality of longitudinal side walls defining a generally trapezoidal cross-sectional area, at least two of said side walls having said depressions therein.

8. A shank as set forth in claim 10 wherein said depressions are each formed as a single shell of a length (u) to width (v) to depth (t) ratio of 12:4:1.